The revival of Doctor Who has been so successful that the BBC has ordered a third series in addition to the second series already planned.

More than 7 million viewers have been gripped by Russell T Davies's version of the cult classic for the past 12 weeks on BBC1, beating rival shows on other channels and making it the most successful TV drama launch in years.

Jane Tranter, head of drama commissioning for the BBC, praised Davies at a screening of the final episode of the first series last night. "Russell is a fantastic writer. Who else could have had the Doctor save the world from a council estate to Downing Street and bring him to the heart of the darkness of reality TV."

Fans of the show should read no further if they do not want details of the plot of Saturday night's episode.

Christopher Eccleston, who has played the Doctor to great acclaim, has confirmed that he will not appear in the new series, and Saturday's show features the dramatic regeneration of his character into a new Doctor, played by David Tennant, star of the BBC's recent adaptation of Casanova.

The episode, which culminates in a battle with an army of thousands of Daleks, also features the first kiss between the Doctor and his assistant.

In a scene in which the life of Rose Tyler, played by Billie Piper, is threatened, the Doctor saves her by engaging her in an apparently passionate embrace.

He then sucks the energy out of her, with a blinding flash of yellow disappearing from Tyler's body into his, and she is saved.

Last night Davies confirmed that he would write six episodes out of 12 in the new series, which will start next March.

There will also be Christmas specials this year and next year, plus the third series, which means in total there are 28 episodes in the pipeline.

Davies said it was his life's ambition to revive Doctor Who and he was astonished at its success. Speaking at the British Academy of Film and Television Arts in London last night, Davies joked that the BBC should not have announced the third series quite so publicly.

Pointing to his agent and his partner, he said; "You could have broken it to them a bit more gently."

He said there would be a series of guest stars in the new episode in the same way that the first series of his revival has featured actors like Simon Callow, Richard Wilson and Penelope Wilton. He also said he was pleased that the series had found new fans.